package order;

/**
 * @Author Janson
 * @Date 2022/2/12 12:29
 * @Version 1.0
 */

import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Stream;

/**
 * 2. 按照字符出现次数对字符串排序
 * 451. Sort Characters By Frequency (Medium)
 *
 * Input:
 * "tree"
 *
 * Output:
 * "eert"
 *
 * Explanation:
 * 'e' appears twice while 'r' and 't' both appear once.
 * So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.
 */
public class Order3_frequencySort {
    public static void main(String[] args) {
        String str = "tree";
        char[] ch = str.toCharArray();
        //1.建立map字典，统计字符串中每个元素出现的次数
        Map<Character, Integer> frequencyForNum = new HashMap();
        for (char c: ch){
            //关键一步，主要功能就是统计字符串中出现相同字符的数目
            frequencyForNum.put(c,frequencyForNum.getOrDefault(c,0)+1);
        }
        //2.建立集合数组，以map中的值为集合数组下表，map的键为集合数组元素
        List<Character>[] frequencyBucket = new ArrayList[ch.length+1];
        for (char c: frequencyForNum.keySet()) {
            int f  = frequencyForNum.get(c);
            if (frequencyBucket[f]==null){
                frequencyBucket[f] = new ArrayList<>();
            }
            frequencyBucket[f].add(c);
        }

        //3.将集合数组从大到小赋值给新集合。因为可能r数组中的某些值为null，故要进行该操作
        /**
         * 用StringBuilder，StringBuffer，List 三种方法存储字符
         * string创建的字符串，创建后不能改变
         * StringBuilder，StringBuffer创建的字符串可以改变，
         * 改变后最终可以用toString()方法转换为不变的字符串，
         *
         * 采用官方提供的方法：append()、insert()、reverse()、setCharAt()、setLength()
         * 区别：StringBuilder线程不安全，执行速度块
         *      StringBuffer 线程安全，执行速度慢
         */
        StringBuilder stringBuilder = new StringBuilder();
        StringBuffer stringBuffer = new StringBuffer();
        List<Character> results = new ArrayList<>();
        for (int i = frequencyBucket.length-1;i >= 0;i--){
            if (frequencyBucket[i] == null)
                continue;
            for (char c: frequencyBucket[i]){
                for (int j=0;j<i;j++){
                    stringBuilder.append(c);
                    stringBuffer.append(c);
                    results.add(c);
                }
            }
        }

        System.out.println("用stringBuilder对象存储字符："+stringBuilder.toString());
        System.out.println("用stringBuffer对象存储字符：" + stringBuffer.toString());
        System.out.println("用list结合存储字符：" + results);
    }
}
